An exploration into diffusion tensor imaging in the bovine ocular lens.

Vaghefi E, Donaldson PJ - Front Physiol (2013)

Bottom Line:
Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions.The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens.This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

Affiliation: Auckland Bioengineering Institute, University of Auckland Auckland, New Zealand ; Department of Optometry and Vision Sciences, University of Auckland Auckland, New Zealand.

ABSTRACTWe describe our development of the diffusion tensor imaging modality for the bovine ocular lens. Diffusion gradients were added to a spin-echo pulse sequence and the relevant parameters of the sequence were refined to achieve good diffusion weighting in the lens tissue, which demonstrated heterogeneous regions of diffusive signal attenuation. Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions. The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens. Up to 30 diffusion gradient directions, and 8 signal acquisition averages, were applied to lenses in culture in order to improve maps of diffusion tensor eigenvalues, equivalent to ADC, across the lens. From these maps, fractional anisotropy maps were calculated and compared to known spatial distributions of anisotropic molecular fluxes in the lens. This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

Figure 10: Comparison of FA map with pattern of large molecule diffusion in the mouse lens. (A) The FA map of the bovine lens from Figure 9D, showing anisotropy in the cortex and relative isotropy in the core. (B) Green fluorescent protein (GFP) expression in an otherwise normal (WT) mouse lens; and in a mouse lens deficient for the Lim2 protein (Lim2Gt/Gt), which is required in the formation of the large molecule diffusion pathway (LMDP; reproduced with permission from Shi et al. (2009). In the normal mouse lens the establishment of the LMDP allows GFP, made by scattered fiber cells, to diffuse evenly in the inner lens, in fibers located 100–200 μm below the lens surface (arrow). In the Lim2-deficient lens the LMDP cannot form and GFP in the inner lens does not diffuse: it remains scattered in the cells that synthesize it. Scale bar, 250 μm.

Mentions:
The DTI methods we have developed here hold potential as new experimental inroads to understanding lens micro-circulation and homeostasis. For example, the FA map presented in Figure 9D shows an intriguing correspondence to a pattern of macromolecular diffusion revealed recently by different methods in the mouse lens (Shi et al., 2009; Figure 10). In the normal mouse lens, a large molecule diffusion pathway (LMDP) is believed to be established between fiber cells located 100–200 μm from the lens periphery. A “tripartite” model for the lens LMDP has been proposed (Shi et al., 2009) in which proteins in the outermost 50 μm of the mouse lens periphery are confined to the cells that synthesize them. Deeper in the lens cortex, where the cells have grown older, proteins diffuse between cells via the established LMDP in a predominantly circumferential direction (i.e., around the visual axis), achieving an even distribution in the tissue (Figure 10B). Near the center of the lens, proteins distribute between cells isotropically. Thus, there appear to be three distinct zones of macromolecular diffusion in the lens.

Figure 10: Comparison of FA map with pattern of large molecule diffusion in the mouse lens. (A) The FA map of the bovine lens from Figure 9D, showing anisotropy in the cortex and relative isotropy in the core. (B) Green fluorescent protein (GFP) expression in an otherwise normal (WT) mouse lens; and in a mouse lens deficient for the Lim2 protein (Lim2Gt/Gt), which is required in the formation of the large molecule diffusion pathway (LMDP; reproduced with permission from Shi et al. (2009). In the normal mouse lens the establishment of the LMDP allows GFP, made by scattered fiber cells, to diffuse evenly in the inner lens, in fibers located 100–200 μm below the lens surface (arrow). In the Lim2-deficient lens the LMDP cannot form and GFP in the inner lens does not diffuse: it remains scattered in the cells that synthesize it. Scale bar, 250 μm.

Mentions:
The DTI methods we have developed here hold potential as new experimental inroads to understanding lens micro-circulation and homeostasis. For example, the FA map presented in Figure 9D shows an intriguing correspondence to a pattern of macromolecular diffusion revealed recently by different methods in the mouse lens (Shi et al., 2009; Figure 10). In the normal mouse lens, a large molecule diffusion pathway (LMDP) is believed to be established between fiber cells located 100–200 μm from the lens periphery. A “tripartite” model for the lens LMDP has been proposed (Shi et al., 2009) in which proteins in the outermost 50 μm of the mouse lens periphery are confined to the cells that synthesize them. Deeper in the lens cortex, where the cells have grown older, proteins diffuse between cells via the established LMDP in a predominantly circumferential direction (i.e., around the visual axis), achieving an even distribution in the tissue (Figure 10B). Near the center of the lens, proteins distribute between cells isotropically. Thus, there appear to be three distinct zones of macromolecular diffusion in the lens.

Bottom Line:
Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions.The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens.This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.

Affiliation:
Auckland Bioengineering Institute, University of Auckland Auckland, New Zealand ; Department of Optometry and Vision Sciences, University of Auckland Auckland, New Zealand.

ABSTRACTWe describe our development of the diffusion tensor imaging modality for the bovine ocular lens. Diffusion gradients were added to a spin-echo pulse sequence and the relevant parameters of the sequence were refined to achieve good diffusion weighting in the lens tissue, which demonstrated heterogeneous regions of diffusive signal attenuation. Decay curves for b-value (loosely summarizes the strength of diffusion weighting) and TE (determines the amount of magnetic resonance imaging-obtained signal) were used to estimate apparent diffusion coefficients (ADC) and T2 in different lens regions. The ADCs varied by over an order of magnitude and revealed diffusive anisotropy in the lens. Up to 30 diffusion gradient directions, and 8 signal acquisition averages, were applied to lenses in culture in order to improve maps of diffusion tensor eigenvalues, equivalent to ADC, across the lens. From these maps, fractional anisotropy maps were calculated and compared to known spatial distributions of anisotropic molecular fluxes in the lens. This comparison suggested new hypotheses and experiments to quantitatively assess models of circulation in the avascular lens.